A new family of materials was disclosed in U.S. Pat. No. 5,043,069 (Bahn et al.), which disclosed the co-extrusion of high temperature polymers with low temperature phosphate glasses and the subsequent injection molding of the glass-polymer materials. The glasses utilized, by necessity, were required to have low softening points which permit low mixing/extrusion temperatures, viz., below about 400.degree. C. Glasses having base compositions within the general zinc phosphate system were found to be especially suitable for the glass component of these glass-polymer alloys; they exhibited low melting temperatures in the 900.degree.-1000.degree. C. range and softening points below about 350.degree. C. Illustrative is that glass composition system disclosed in U.S. Pat. No. 4,940,677 (Beall et al.).
The usual process in the prior art preparation of these zinc-phosphate glasses comprised mixing selected batch materials together in desired proportions and heating the mixture to elevated temperatures to form a melt from which glass particles could thereafter be formed. The resulting glass particles were thereafter mixed with the desired polymer in accordance with the Bahn et al. disclosure and processed to form the glass polymer alloys.
Typically in the past, the batch material source of phosphate in these glasses was monoammonium phosphate. The disadvantage in using this conventional source of phosphate is that environmental concerns dictate the necessity of using pollution control systems to collect the ammonia byproduct. Furthermore, monoammonium phosphate as a batch material is quite costly, when compared to other sources of phosphate such as phosphoric acid. Therefore, methods of incorporating the phosphate into the batch without the use of monoammonium phosphate were sought.
Although there are commercially available forms of zinc phosphate which can be used in the batching zinc phosphate glasses, the zinc-to-phosphate ratio exhibited by these materials is not appropriate for forming the glasses disclosed in the above-referenced Beall et al. patent. Specifically, these sources were not rich enough in phosphate in order to form those glasses; i.e., in utilizing these commercially available zinc phosphate sources, the required amount of zinc batch material is met prior to that of the phosphate. This, in turn, would require an additional source of phosphate, i.e., most likely monoammonium phosphate. Thus, there would be no advantage in utilizing these materials as the source of the phosphate.